Carbon nanotubes (CNTs) and inorganic nanowires (INWs) have been explored for potential applications in nanoelectronics, lasers, field emission devices, displays, chemical and biosensors, detectors and various other nanodevices. In order to realize the potential of CNTs and INWs, the major manufacturing issue now is lack of control of the size (diameter and height) and hence the properties being exploited for the application under consideration.
For example, nanotubes of different diameters will have different bandgaps, electronic, thermal and other properties. In addition to size, orientation becomes important in many applications. A thin film of single-walled carbon nanotubes (SWNTs), where the structure looks like spaghetti, does not have much value in practice. Most of the properties such as electrical and thermal conductivities are high in the axial direction relative to the other directions. Therefore, a film with a random weave of nanotubes often exhibits very poor properties. In the case of nanowires, random orientation on a substrate is typically not suitable for device manufacture as the wires are hard to contact and lack uniformity of density across the wafer surface. In addition to control of diameter and orientation, positional control is also important, regardless of whether a single or multiple CNTs or INWs are in communication between metal electrodes for any type of nanodevice. In many cases, vertical orientation of CNTs or INWs of a specific diameter and height at pre-selected locations is desirable to achieve maximum yield of the device.
A good example for orientation/position/diameter control requirement is the vertical transistor (using silicon or germanium or CNT) with a surround gate. Early demonstrations necessarily used micron long NWs; but to be in step with Moore's law scaling and beyond, the source-to-drain separation and hence the nanowire height have to be under 50 nm now and all the way down to a few nm in the future. Therefore, the height also has to be controlled. Another example is a nanoelectrode array consisting of uniform diameter/height carbon nanofibers for biosensing applications such as lab-on-a-chip, pathogen detection, environmental monitoring etc. Here, each of the carbon nanofibers (CNF) is functionalized with a probe (DNA, mRNA . . . ) suitably selected to hybridize with a target. Signal detection is done electrochemically and therefore each CNF is located just far away from its neighbor to avoid the overlap of the radial diffusion layers and thus crosstalk between neighboring electrodes. Several other examples can be cited such as lasers, detectors, displays, etc., wherein control of diameter, height, position and orientation of CNTs and INWs is critical in manufacturing.
Chemical vapor deposition (CVD) has been successfully used to grow CNTs on patterned substrates, which is the first step towards manufacturing. This catalyzed CVD is similar to the VLS process for the growth of INWs. In all of the above cases, the catalyst must be available in the form of nanosize particles to facilitate CNT and INW growth. Careful analysis has also confirmed a tight correlation between the particle size and the resulting tube or wire diameter. When the catalyst grain size is large or in the form of a smooth thin film, NT/NW growth does not happen or, at best, the growth is sparse. A common approach to catalyst preparation is sputtering or evaporation of the requisite metal into a thin film 1-20 nm in thickness. This is a quick process and amenable to produce patterned wafers. At the growth temperature, the thin film breaks into tiny droplets which serve as the nucleation centers. This is the reason why an inverse correlation between the nanowire density and melting point of the catalyst metal has been reported. Note that the molten metal droplet serves as a ‘soft-template’ for the nanowire growth. Therefore, the easier the metal melts, the higher will be the growth density.
A major drawback of the thin film approach for catalyst preparation is that the droplet size distribution upon melting of the film is Gaussian. As a result, the resulting NW or CNT diameter distribution would be Gaussian as well. Note that thicker tubes and wires grow slower and hence the diameter distribution in growth would translate into a height variation as well. In addition, melting of the metal film across a wafer gives no positional control of the nanotubes or nanowires. A typical outcome of this approach is a forest of nanotubes and nanowires where diameter and height variations are evident. Even when lithography is used to pre-select the position of the NW growth, the diameter and height can change due to a change in particle size during growth. An alternative approach in the literature has been the use of monodispersed metal colloids such as gold particles. But the particle size and its position cannot be guaranteed once the substrate heating begins. The particles migrate laterally and the size also can change either due to coalescence or breakup.
In summary, control of position and diameter in CNT and INW growth has been elusive to date. What is ideally required is a “virtual fence” around each catalyst particle to arrest its migration from the original position, avoid agglomeration or breakup and retain the original size. One method of remedying this manufacturing issue, as disclosed herein, is a modified Atomic Force Microscope (AFM) approach, which enables large-scale and higher throughput and fabrication of NWTs and INWs having prescribed and uniform diameters, height and positional characteristics.